This invention relates to an optical disk on which information is recorded in pits with high density and an optical disk apparatus containing the optical disk and a reproduction optical system.
With the recent advances in image digital signal processing techniques and moving-picture compression techniques, the latter of which have been developed by such a standardizing organization as the MPEG (Moving Picture Image Coding Experts Group), there is a growing expectation of the advent of an optical disk capable of reproducing moving-picture information such as a movie for two hours and being the same size as a CD (compact disk) in place of a VTR or laser disk. The recording capacity required to record two hours of moving-picture information in the form of analog video signals by a standard TV system such as NTSC as on the laser disk, amounts to 80 Gbyte including sound. Use of moving-picture compression techniques prescribed by a standardized method called MPEG-2, for example, requires as small a capacity as nearly 4 Gbyte even for a picture quality as good as a high picture-quality VTR such as S-VHS. The 4-Gbyte disk has been put into practical use in the form of a 300-mm diameter write-once read-many optical disk. As more and more optical disks will be used in homes in the future, it is needed to achieve an easy-to-use 120-mm diameter disk which has the same size and almost the same capacity as the CD.
The capacity of the CD format presently available as the music CD or the CD-ROM is 790 Mbyte at the maximum (when the linear velocity is 1.2 m/s). The capacity of this order can store only 24 minutes of compressed moving-picture information by MPEG-2. Thus, to store two hours of compressed moving-picture information by MPEG-2 with the CD size, the recording density must be made five times as high as that of the CD. In the current CD format, the substrate thickness is 1.2 mm, the track pitch is 1.6 xcexcm, the pit pitch is 1.66 xcexcm when the linear velocity (relative velocity between light beam and disk=disk""s circumferential velocity) is 1.2 m/s, the bit length is 0.59 xcexcm, and the modulation method is EFM (eight to fourteen modulation). In the reproduction optical system, the reproduction semiconductor laser, or the laser diode (LD) has a wavelength of 780 nm, the object lens has an NA (numerical aperture) of 0.45, and the beam spot has a diameter of 1.4 xcexcm. The beam spot diameter is selected mainly from the standpoint of avoiding the effect of cross talk between adjacent tracks.
To increase the recording density of the optical disk requires techniques for forming small pits in the disk and those for making the beam spot size small on the optical disk in the reproduction optical system. Concerning techniques for forming pits, for example, an optical disk matrix recording technique using Kr ion laser light (ultraviolet rays) with a wavelength of 351 nm has been proposed (The 1993 Autumn National Convention of the Applied Physics Society, 28-SF-2). This technique makes it possible to form smaller pits than a conventional Ar ion laser. In the reproduction optical system, by making the wavelength of the reproduction laser beam shorter and increasing the NA, the beam spot diameter can be made smaller. Actually, however, with conventional techniques used in CD players, even if a short wavelength light source such as a red laser diode were used, the capacity would be increased by 1.5 times at most. With such an increase in the capacity, it cannot be expected to increase the capacity by five times that of an ordinary CD, which is what is required to record two hours of compressed moving-picture information.
As described above, with the conventional optical disk techniques, to avoid the problem of cross talk between adjacent tracks, the track pitch and pit pitch are set larger than the beam spot diameter of the reproduction light beam. As a result, only by making the wavelength of reproduction light beam shorter and increasing the NA of the object lens, the recording density cannot be raised to the extent that the capacity required to store two hours of compressed moving-picture information by MPEG-2 with the CD size, for example.
It is an object of the present invention to provide an optical disk and an optical disk apparatus which can lessen cross talk between adjacent tracks to the extent that there is no problem in practical use, even if the track pitch and pit pitch are smaller than the beam spot diameter of the reproduction light beam, and which achieves a higher density and a greater capacity than in the prior art.
It is another object of the present invention to provide a optical disk and an optical disk apparatus which enable a high density recording in a tilt angle of the optical disk which is less than 10 mrad.
According to the present invention, there is provided an optical disk comprising: a substrate in which information to be picked up by a light beam radiated thereto through an objective lens are recorded as pit arrays formed at a predetermined track pitch; and a reflective layer formed on the substrate, the track pitch being set within a range of
(0.72 to 0.8)xcex1xc3x97(xcex/NA)/1.14 xcexcm,
when the light beam has a wavelength of xcex nm and the objective lens has a numerical aperture of NA, the xcex1 being represented by:
xcex1=3.617xc3x9710xe2x88x9214xc3x97(xcex8A ds/xcex)3+1.248xc3x9710xe2x88x929xc3x97(xcex8A ds/xcex)2+0.865
when the optical disk has a thickness of ds xcexcm, and an allowable tilt angle of xcex8A, and the allowable tilt angle xcex8A being not more than 7 mrad.
According to the present invention, there is provided an optical disk apparatus comprising: an optical disk including a substrate in which information are recorded as pit arrays formed at a predetermined track pitch, and a reflective layer formed on the substrate; an objective lens arranged to face the optical disk; a light beam generator which radiates a light beam onto the optical disk through the objective lens; and a reproduction unit which detects a reflected beam of the light beam radiated onto the optical disk by the light beam generator so as to reproduce the information recorded in the optical disk, the track pitch being set within a range of
(0.72 to 0.8)xcex1xc3x97(xcex/NA)/1.14 xcexcm,
when the light beam has a wavelength of xcex nm and the objective lens has a numerical aperture of NA, the xcex1 being represented by:
xcex1=3.617xc3x9710xe2x88x9214xc3x97(xcex8A ds/xcex)3+1.248xc3x9710xe2x88x929xc3x97(xcex8A ds/xcex)2+0.865
when the optical disk has a thickness of ds xcexcm, and an allowable tilt angle of xcex8A, and the allowable tilt angle xcex8A being not more than 7 mrad.
According to the present invention, there is provided an optical disk comprising: a pair of transparent substrates which face each other and in which information to be reproduced in response to a light beam radiated thereto through an objective lens are recorded as pit arrays formed at a predetermined track pitch; and a pair of reflective layers formed on those sides of the transparent substrates which face each other, the track pitch being set within a range of
(0.72 to 0.8)xcex1xc3x97(xcex/NA)/1.14 xcexcm,
when the light beam has a wavelength of xcex nm and the objective lens has a numerical aperture of NA, the xcex1 being represented by:
xe2x80x83xcex1=3.617xc3x9710xe2x88x9214xc3x97(xcex8A ds/xcex)3+1.248xc3x9710xe2x88x929xc3x97(xcex8A ds/xcex)2+0.865
when the optical disk has a thickness of ds xcexcm, and an allowable tilt angle of xcex8A, and the allowable tilt angle xcex8A being not more than 7 mrad.
According to the present invention, there is provided an optical disk comprising: a substrate in which information to be reproduced in response to a light beam radiated thereto through an objective lens are recorded as pit arrays formed at a predetermined track pitch; and a reflective layer formed on the substrate, the track pitch being set within a range of
(0.72 to 0.8)xcex1xc3x97(xcex/NA)/1.14 xcexcm,
when the light beam has a wavelength of xcex nm and the objective lens has a numerical aperture of NA, each of pits of the pit arrays being a trapezoidal-section pit which has an upper-region width within a range of (0.3 to 0.50)xcex1xc3x97(xcex/NA)/1.14 xcexcm and a lower-region width within a range of (0.2 to 0.32)xcex1xc3x97(xcex/NA)/1.14 xcexcm, and the xcex1 being represented by:
xcex1=3.617xc3x9710xe2x88x9214xc3x97(xcex8A ds/xcex)3+1.248xc3x9710xe2x88x929xc3x97(xcex8A ds/xcex)2+0.865
when the optical disk has a thickness of ds xcexcm, and an allowable tilt angle of xcex8A.
According to the present invention, there is provided an optical disk apparatus comprising: an optical disk including a substrate in which information are recorded as pit arrays formed at a predetermined track pitch, and a reflective layer formed on the substrate; an objective lens arranged to face the optical disk; means for radiating a light beam onto the optical disk through the objective lens; and means for detecting a reflected beam of the light beam radiated onto the optical disk by the radiating means so as to reproduce the information recorded in the optical disk, the track pitch being set within a range of
(0.72 to 0.8)xcex1xc3x97(xcex/NA)/1.14 xcexcm,
when the light beam has a wavelength of xcex nm and the objective lens has a numerical aperture of NA, each of pits of the pit arrays being a trapezoidal-section pit which has an upper-region width within a range of (0.3 to 0.50)xcex1xc3x97(xcex/NA)/1.14 xcexcm and a lower-region width within a range of (0.2 to 0.32)xcex1xc3x97(xcex/NA)/1.14 xcexcm, and the xcex1 being a multiplication ratio that ensures an allowable disk tilt angle when the wavelength is 0.65 xcexcm, and being represented by:
xcex1=3.617xc3x9710xe2x88x9214xc3x97(xcex8A ds/xcex)3+1.248xc3x9710xe2x88x929xc3x97(xcex8A ds/xcex)2+0.865.
According to the present invention, there is provided an optical disk comprising: a substrate in which information to be reproduced in response to a light beam radiated thereto through an objective lens are recorded as pit arrays formed at a predetermined track pitch; and a reflective layer formed on the substrate, the track pitch being set within a range of
(0.72 to 0.8)xcex1xc3x97(xcex/NA)/1.14 xcexcm,
when the light beam has a wavelength of xcex nm and the objective lens has a numerical aperture of NA, each of pits of the pit arrays being a trapezoidal-section pit which has an upper-region width within a range of (0.3 to 0.50)xc3x97( xcex/NA)/1.14 xcexcm and a lower-region width within a range of (0.2 to 0.32)xc3x97(xcex/NA)/1.14 xcexcm, and the xcex1 being a multiplication ratio that ensures an allowable disk tilt angle when the wavelength is 0.65 xcexcm, and being represented by:
xcex1=3.617xc3x9710xe2x88x9214xc3x97(xcex8A ds/xcex)3+1.248xc3x9710xe2x88x929xc3x97(xcex8A ds/xcex)2+0.865.
According to the present invention, there is provided an optical disk apparatus comprising: an optical disk including a substrate in which information are recorded as pit arrays formed at a predetermined track pitch, and a reflective layer formed on the substrate; an objective lens arranged to face the optical disk; a light beam generator which radiates a light beam onto the optical disk through the objective lens; and a reproduction head which detects a reflected beam of the light beam radiated onto the optical disk by the light beam generator so as to reproduce the information recorded in the optical disk, the track pitch being set within a range of
(0.72 to 0.8)xcex1xc3x97(xcex/NA)/1.14 xcexcm,
when the light beam has a wavelength of xcex nm and the objective lens has a numerical aperture of NA, each of pits of the pit arrays being a trapezoidal-section pit which has an upper-region width within a range of (0.3 to 0.50)xc3x97(xcex/NA)/1.14 xcexcm and a lower-region width within a range of (0.2 to 0.32)xc3x97(xcex/NA)/1.14 xcexcm, and the xcex1 being a multiplication ratio that ensures an allowable disk tilt angle when the wavelength is 0.65 xcexcm, and being represented by:
xcex1=3.617xc3x9710xe2x88x9214xc3x97(xcex8A ds/xcex)3+1.248xc3x9710xe2x88x929xc3x97(xcex8A ds/xcex)2+0.865.
By setting various parameters of the pit shape at the above-described values, the amount of cross talk between adjacent tracks is suppressed to less than xe2x88x9220 dB, which must be met to restore the original information from the reproduced signal, and the reproduced signal level and the level of the push-pull signal for tracking are maintained sufficiently.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.